Fig:Fully refrigerated LPG carrier at sea
The only other cargoes allowed to be carried simultaneously with Propylene oxide P.O. are propane and butane.
Propylene oxide P.O. must only be loaded and unloaded directly, i.e. with this plant it must not:
i)Be loaded by the compressors, or
ii) Heated with the heat exchangers or cooled by the compressors.
This is affected by removing spool pieces so that there is only a liquid line into the tanks and a gas line
out. All other lines and plant, is then isolated.
Extract from IMO regulations
(a) Flame Screens on Vent Outlets
Cargo tank vent outlets are to be provided with readily renewable and effective flame screens or safety
heads of an approved type when carrying a cargo referenced to this section. Due attention is to be
paid to the design of the flame screens and vent heads to the possibility of the blockage of these
devices by the freezing of cargo vapour or by icing up in adverse weather conditions. Ordinary
protection screens are to be fitted after removal of the flame screens.
(b) Maximum Allowable Quantity of Cargo per Tank
When carrying a cargo referenced to this section, the quantity of the cargo is not to exceed 3,000 cu
mtrs in any one tank, (tank means tank compartment).
(c) Propylene Oxide and Mixtures of Ethylene Oxide-Propylene Oxide with Ethylene
Oxide content of not more than 30% by weight
i )Products carried under the provisions of this section must be acetylene-free.
ii) Unless cargo tanks are properly cleaned, these products are not to be carried in tanks which have
contained as one of the three previous cargoes any product known to catalyse polymerisation, such as:
Ammonia, anhydrous and ammonia solutions;
Amines, and amine solutions;
Oxidising substances, e.g. chlorine.
- Before loading, the tanks are to be thoroughly and effectively cleaned to remove all traces of
previous cargoes from tanks and associated pipework, except where the immediate prior cargo has been
P.O. or P.O./E.O. mixtures.
- In all cases, the effectiveness of cleaning procedures for tanks and associated acidic or alkaline
materials remain that might create hazardous situations in the presence of these products.
- Tanks must be entered and inspected prior to each initial loading of these products to ensure
freedom of contamination, heavy rust deposits and any visible structural defects. When cargo tanks
are in continuous service for these products, such inspections are to be performed at intervals of not
more than two years.
- Tanks for the carriage of these products must be of steel or stainless steel construction.
- Tanks which have contained these products may be used for other cargoes after thorough
cleaning of tanks and associated pipework systems by washing or purging.
- All valves, flanges, fittings, and accessory equipment must be of a type suitable for use with
these products and are to be constructed of steel or stainless steel or other material acceptable to the
Administration. The chemical composition of all material used is to be submitted to the Administration
for approval prior to fabrication. Discs or disc faces, seats and other wearing parts of valves must be
made of stainless steel containing not less than 11% chromium.
- Gaskets must be constructed of materials which do not react with, dissolve in, or lower the
auto-ignition temperature of these products and which are fire-resistant and possess adequate
mechanical behaviour. The surface presented to the cargo should be POLYTETRAFLUORETHYLENE
(PTFE) or materials given a similar degree of safety by their inertness. Spirally wound stainless steel
with a filler of PTFE or similar fluorinated POLYMER may be accepted by the Administration.
Spirally wound gaskets are required to be fitted to cargo tank lids by the U.S.C.G.
- The following materials are generally found unsatisfactory for gaskets, packing and similar uses
in containment systems. These products would require testing before being approved by the
Administration:
- NEOPRENE or NATURAL RUBBER, if it comes in contact with the products;
- ASBESTOS or binders used with ASBESTOS;
- Materials containing OXIDES of MAGNESIUM, such as mineral wools.
- Filling and discharge piping is to extend to within 100 mm of the bottom of the tank or any
sump pit.
- The products are to be loaded and discharged in such a manner that venting of the tanks to
atmosphere does not occur. If vapour return to shore is used during tank loading, the vapour return
system connected to a containment system for the product is to be independent of all other
containment systems.
- During discharging operations, the pressure in the cargo tank must be maintained above 0.07
bar gauge.
- The cargo is to be discharged only by deep well pumps, hydraulically operated submerged
pumps, or inert gas displacement. Each cargo pump is to be arranged to ensure that the product does
not heat significantly, if the discharge line of the pump is shut off or otherwise blocked.
- Tanks carrying these products must be vented independently of tanks carrying other products.
Facilities are to be provided for sampling the tank contents without opening the tank to atmosphere.
- Cargo hoses used for transfer of these products must be marked.
“ FOR ALKALINE OXIDE TRANSFER ONLY ”
- Hold spaces are to be monitored for these products.
- Prior to disconnecting shore-lines, the pressure in liquid and vapour lines must be relieved
through suitable valves installed at the loading header. Liquid and vapour from these lines is not to be
discharged to atmosphere. All connections must be purged with nitrogen.
- Pressure relief valve settings must not be less than 0.2 bar gauge and for type C independent
cargo tanks not greater than 7.0 bar gauge for the carriage of propylene and not greater than 5.3 bar
gauge for the carriage of Ethylene Oxide-propylene Oxide mixtures.
- The piping system for tanks to be loaded with these products is to be completely separated
from the piping systems for all other tanks, including empty tanks, and from all cargo compressors. If
the piping system for the tanks to be loaded with the product is not independent, the required piping
separation must be accomplished by the removal of spool pieces, blank flanges, or other pipe sections
and the installation of blank flanges at these locations. The required separation applies to all liquid and
vapour piping, liquid and vapour vent lines and any other possible connections such as common inert
gas supply lines.
- The products are to be transported only in accordance with the cargo handling plans that have
been approved by the Administration. Each intended loading arrangement is to be shown on a
separate cargo handling plan. Cargo handling plans must show the entire cargo piping system and the
locations for installation of blank flanges needed to meet the above piping separation requirements. A
copy of each approved cargo handling plan is to be kept on board the ship. The Certificate of fitness
for the Carriage of Liquefied Gases in Bulk is to be endorsed to include reference to the approved
handling plan.
- Before each initial loading of these products, and before every subsequent return to such
service, certification verifying the required piping separation has been achieved is to be obtained from a
responsible person acceptable to the port administration and carried on board the ship. Each
connection between a blank flange and pipeline flange must be fitted with wire, and sealed by the
responsible person to ensure that inadvertent removal of the blank flange is impossible.
The “responsible person” may be, for example, the ship’s master or the society’s local surveyor.
- The maximum allowable tank filling limits for each cargo tank are to be indicated for each
loading temperature which may be applied and for the applicable maximum reference temperature, on
a list to be approved by the Administration. A copy of the list is to be permanently kept on board by
the master.
- The cargo must be carried under a suitable protective padding of nitrogen gas. An automatic
nitrogen make-up system is to be installed to prevent the tank pressure falling below 0.07 bar gauge in
the event of product temperature falling due to ambient conditions or malfunctioning of the
refrigeration system. Sufficient nitrogen must be available on board to satisfy the demand of the
automatic pressure control. Nitrogen of commercially pure quality (99.9% by volume) is to be used for
padding. A battery of nitrogen bottles connected to the cargo tanks through a pressure reduction valve
satisfies the intention of the expression “Automatic” in this context.
- The cargo tank vapour space must be tested prior to and after loading to ensure that the
oxygen content is 2% by volume or less.
- A water spray system of sufficient capacity is to be provided to blanket effectively the area
surrounding the loading manifold, exposed deck piping associated with product handling and the tank
domes. The arrangement of piping and nozzles is to be such as to give a uniform distribution rate of
10 litres / sq metre / minute. The water spray system is to be capable of both local and remote
manual operation and the arrangement should ensure that any spilled cargo is washed away.
Additionally, a water hose with pressure to the nozzle, when ambient temperatures permit, shall be
connected ready for immediate use during loading and unloading operations.
For ships trading to the territorial waters of the USA the relevant requirements of the USCG must be
complied with, i.e. the water spray system required by 17.20.17 must operate automatically in case of
fire.
Loading P.O. / P.O. - E.O. Mixes
The following conditions must be observed before loading P.O.:
(a) P.O. must not be loaded into tanks which contained during one of the last three voyages either
ammonia, amines, caustic solutions or other products above to react with P.O. A proof of the last ten
cargoes must be kept on board.
(b) Washing water used in the tanks must not contain dissolvents or additives which have not been
authorised by a competent technical supervision authority. The last washing has to be done with clear,
pure water. Samples of the last purge water are to be handed to the responsible person or to the
terminal before loading.
(c) The ship must have her own independent nitrogen system for padding the tank after loading.
There is to be sufficient nitrogen available on board to guarantee a minimum tank pressure of 5 psi,
(0.35 bar).
(d) Sealing material has to be approved and controlled by the competent technical supervision
authority. All tanks and pipes are to be clean, dry and prepared for inspection after arrival at the
loading port. Before loading the tanks and pipes have to be inspected and approved by the responsible
person of the shore terminal or by the marine inspector.
(e) All air must be purged from the tanks and pipes with pure nitrogen or inert gas, so that the
oxygen content is <0.3%, and the carbon dioxide content <1%.
On board generated Co2 must not be used.
The quality of the inert gas must be confirmed by laboratory analysis and approved by the responsible
person of the shore terminal.
(f) All open connections or pipes and valves must be closed or equipped with blind flanges.
Loading hoses must be approved and marked by the responsible authorities.
They must not be used for any other product.
(g) The pipe systems to be used for loading cargo, including the return lines for liquids and gases
must be entirely segregated from the main cargo handling system on board ship.
(h) Cargo tanks must not be filled to more than 98% of the tank capacity, relative to the density of
the gas at the set pressures of the safety valves.
For P.O./E.O. mixtures the pressure must be not greater than 5.3 barg.
The maximum volume to which tanks may be filled is calculated using the following formula:
VL = 0.98V pr/pl
Where
VL = Maximum volume to which tank may be loaded.
V = Volume of the tank.
PR = Relative density of cargo at the reference temperature.
PL = Relative density of cargo at the loading temperature and pressure.
NOTE : Reference temperature defined as per paragraph 15.1.4 IMO
code.
(i) On completion of cargo loading, and after blowing liquid residues from the piping system
ashore with nitrogen, the system must be purged with nitrogen from a shore supply or the on board
bottles. Finally the tank nitrogen padding is introduced.
Transport of P.O. / P.O. - E.O. MIXES
P.O. must be transported under a padding of nitrogen, and because it has a relatively high boiling
temperature of +34°C at atmospheric pressure, pressure in the tanks should be at least 0.35 barg
above atmospheric. Pressure is to remain constant throughout the loaded voyage by feeding nitrogen
as required. A nitrogen supply system is installed for this purpose.
Safety valve setting for transportation of P.O. / P.O. mixes must be in accordance with gas plant
supplier’s instruction manual, usually USCG requirements.
Discharging
During discharging operations the following conditions must be observed:
(a) The tanks and piping system must be segregated from all other cargo systems on board ship.
(b) Over-pressure in the tank must not fall below 0.07 bar. Unloaded liquid is replaced with inert gas
or nitrogen from shore.
(c) Cargo is discharged using deep well pumps and possibly the boosters. Total discharge can be
achieved by displacement with nitrogen and by using the 2” stripping line. Draining and purging of lines
requires a high purge gas speed, which is best obtained by using the nitrogen accumulated in the tank.
(d) Liquid residues in the deck lines are to be drained into the collectors and discharged ashore.
(e) When changing cargo grades, tanks must be totally discharged, water washed and ventilated for
inspection.
The inert gas from the stripping phase can be retained in the tanks because washing must be done in an
inerted atmosphere. This is to offset the possible formation of static charges during washing, due to the
relatively high boiling temperature of the cargo.
(f) Tank washing is to be carried out in accordance with the approved Annex II Procedures and
Arrangements manual, a copy of which is retained on board the ship.
In the context of the requirements of MARPOL Annex II, provided the tanks have been
properly stripped and ventilated dry, any water introduced into the tank for preparing the
tank to receive the next cargo can be regarded as being clean, and not subject to the
discharge restrictions included in Annex II.
*The water for tank washing is supplied from ashore or from an on board soft water generator and will
normally be at ambient temperature. Pressure at the spray line connection is to be at least 3 barg
above tank pressure. (At 3 bar the flow rate will be approximately 28 cu mtrs/hr/tank). Water can be
pumped from the bottom of the tanks by deep well pumps into adjacent tanks, or recirculated, and
finally ashore, or discharged to sea. This procedure is to be repeated several times.
After completely drying and ventilating tanks and pipe systems, the system shall be inspected and the
appropriate certificates issued.
Related Information:
- Tackling fire onboard LNG & LPG ships
- Detail guideline for Ballast operation at sea by LPG carrier
- Handling cargo related documents for LPG carrier
- Cargo sampling procedure for liquefied gas cargo
- Cargo measurement and calculation guideline for LPG carriers
